When I load any of the double bass articulations into a Dorico project, an F#0 always plays at the beginning, and yet no notes or key switches have been added. This doesn't happen with any other instruments, so could it be a bug?

If you are using an expression map, I would try temporarily removing it. This sounds similar to an issue I had where a note is expected to be a key switch, but is not in the instrument’s key switch map and it therefore plays.